The reaction products of rosin, polyhydric alcohols, formaldehyde and phenols (hereinafter "phenolic-modified rosin esters") have been known for over 60 years. See, e.g., U.S. Pat. No. 2,007,983 to Rosenblum. They are typically prepared by the reaction of rosin (a mixture of isomeric C.sub.20 tricyclic mono-carboxylic acids) with a formaldehyde-containing compound, and a phenolic compound. This phenolic resin is then esterified with a polyhydric alcohol (hereinafter "polyol") providing the phenolic-modified rosin esters. These materials are typically dark colored and of moderately high molecular weight. Phenolic-modified rosin esters currently find widespread commercial use as binders for offset printing inks. See, e.g., Roger E. Burke, "Rosin-based Printing Inks", Naval Stores, Chapter 10. Pulp Chemicals Association (1989).
The prior art contains numerous description of phenolic-modified rosin esters which are reportedly useful in offset printing inks. Exemplary are U.S. Pat. No. 4,398,016 to Homma et al. and U.S. Pat. No. 4,391,640 to Okoshi et al., which are both directed to processes for producing phenolic-modified rosin esters, which are further modified with an animal or vegetable oil. U.S. Pat. No. 4,002,585 to Oishi et al. discloses a reaction product of rosin, an .alpha.,.beta.-ethylenically unsaturated carboxylic acid-modified petroleum resin, a resole-type phenol-formaldehyde pre-condensate and a polyhydric alcohol having a high softening point and high solubility in a hydrocarbon solvent. U.S. Pat. No. 4,857,624 to DeBlasi at al relates to a reportedly inexpensive process for producing a phenolic-modified rosin ester which exhibits high hold-out, excellent printability and excellent reducibility. U.S. Pat. No. 4,744,925 relates to a method of preparing esters of rosin, wherein rosin and polyhydric alcohol are reacted in the presence of phosphinic acid and a phenol sulfide compound, as combined catalyst, to reduce the reaction time for ester formation and to provide a rosin ester of improved color and oxidative stability; however, no particular utility is reported for the rosin esters disclosed in the '925 patent.
Phenolic-modified rosin esters are seldom employed as tackifiers. In part, this is probably due to the very dark color and high molecular weight of phenolic-modified rosin esters. Currently, rosin esters (made without phenol or formaldehyde modification) and terpene phenolics (made from terpenes and phenols) are commercially used as tackifiers, and these materials have colors, as measured on the Gardner scale, of less than about 6, where lower numbers correspond to lighter-colored resins. In contrast, phenolic-modified rosin esters typically have Gardner colors in excess of 10. Thus, color alone effectively precludes phenolic-modified rosin esters from being employed as commercially viable tackifiers.
In addition, since phenolic-modified rosin esters have been primarily developed for use in offset printing inks, a critical property for these esters is oil solubility. Offset printing inks contain a large amount of hydrocarbon solvent, and therefore the binder resins must have good solubility in these solvents. This particular property is largely irrelevant for a tackifier resin.
There is a continuing need for new and improved tackifier resins, which meet the basic requirements of low color, good compatibility with elastomers, and reasonable manufacturing costs. The present invention fulfills this need and provides related advantages as described herein.